For some time, compounds which may be comprised by the term "prostacyclins" have attracted pharmacological and medical interest. Prostacyclin, or PGI.sub.2, which is a recently isolated natural material belonging to the family of prostaglandins, is distinguished by pronounced properties of inhibiting platelet aggregation (The Lancet 1977, 18). The physiological effects of prostaglandins and prostacyclins are, however, both in vitro and in the mammalian body, of short duration since they are rapidly converted into pharmacologically inactive metabolites. Furthermore, it is a disadvantage that these compounds have, in addition to the desired physiological effect, at the same time a number of undesired physiological side effects which greatly restrict their use as medicaments.